Reprints 1-50

Date:
1950-1963
Reference:
PP/CRI/M/1/1
Catalogue details

Licence: In copyright

Credit: Reprints 1-50. Source: Wellcome Collection.

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    46 F. H. C. Crick and A.F.W. Hughes Fig. 2. (B-H) Curve of oxidised iron, (a) and (b) are recoil curves. us that if the powder is dried it oxidises visibly, so that as we used it the material was substantially oxidised iron. An analysis by Messrs. Geochemical Laboratories has confirmed this. The percentage composition is as follows: Iron 61.0 Aluminium 5.4 Combined oxygen 28.3 Water 2.1 Silica, manganese 2.2 The B/H curve was found by pressing the powdered material into a glass tube so that it formed a cylinder 15 cm. long and 0.4 cm. diameter. The length/diameter ratio was therefore so great that demagnetising effects could be neglected. The method used was the ballistic one, a flux coil being wound on the glass tube, and the whole placed inside a long solenoid. The curve of (B-H) V H is given in Figure 2. The values have been corrected for the effect of packing, by increasing the (B-H) figures in the ratio of The physical properties of cytoplasm 47 apparent density 4.2 ^ ^ true density 2.0 so that they should represent average values for individual particles. Owing to the low permeability the results are not very accurate, but they are good enough to show the essential features of the curve. The permeability is low (maximum about 3.6), and the material saturates at (B-H) equal to about 1,500. The back part of the curve shows that the effective value of (B-H) for ellipsoids of different length/diameter ratios does change, but not enormously. Length/diameter ratio 1.0 2.0 3.0 5.0 °o Effective (B-H) 225 325 390 460 550 The effective value of the slope of the curve over the part which interests us, is about 4 J. The recoil curves show that a field of 50 oersteds applied in reverse momentarily to a short particle (axial ratio 2:1) may permanently lower its magnetic moment, probably by about 20 per cent. It would there fore be inadvisable to use fields much greater than this. Fields of this mag nitude would not give exactly the usual variation of couple as cos 0 (where 6 is the angle between the applied field and the magnetic axis of the particle), but for our purposes the differences are not important. 4. Methods of producing magnetic fields Our first method consisted in mounting a large U-shaped Alcomax magnet - horizontally with the objective in between the poles. The size of the faces was 1.9 x2.2 cms and the distance between them 2.2 cm. The mean length of the U was 13 cm. The bottom edge of the magnet was a few millimeters above the level of the culture. This magnet could slide horizontally in and out, so that the strength of the field could be varied from about 800 oersteds to 20 oersteds. It could also be rotated about the objective to change the direction of the field, over an arc of about 150°. This magnet was used for the original experiments with magnetite. By simple mechanical means the magnet was made to perform oscillations of ±15°, with periods of the order of 5 to 15 seconds. This was done by means of an electric motor, a reduction gear, and a series of levers. This magnet was also used for the experiments on prodding, where a large field was re